1. Field of the Invention
The invention relates to an image forming apparatus of an electrophotographic system.
2. Related Background Art
An image forming apparatus of an electrophotographic system in which an electrostatic latent image is formed by exposing the surface of a photosensitive drum, the electrostatic latent image is developed, toner (developing material) is supplied onto the photosensitive drum through a developing roller, and a toner image is formed has been disclosed in JP-A-2002-365890.
According to the image forming apparatus of JP-A-2002-365890, when an image ratio called a print duty is high, in order to avoid a white-on-black phenomenon of an image which is caused by a lack of supply amount of the toner, a rotational speed of a developing material agitation conveying member to convey the toner to the developing roller while agitating it is raised in accordance with the print duty, thereby increasing the supply amount of the toner and preventing the occurrence of the white-on-black phenomenon of the image.
According to the above conventional image forming apparatus, the toner which is supplied by the developing material agitation conveying member is formed in a layer shape on the developing roller and the toner is supplied to the photosensitive drum through the developing roller. At this time, by making the toner-deposited developing roller come into pressure contact with a toner restricting member while rotating such a developing roller, it is restricted so that the toner of a thickness over a predetermined thickness dimension is not deposited onto the developing roller, and the toner layer of the desired thickness dimension is formed.
However, in the above conventional image forming apparatus in which the toner restricting member is come into pressure contact with the developing roller and the toner amount is restricted, there occurs inconvenience called filming in which, although the amount of toner which is supplied to the developing roller is sufficient, in the case of forming an image of a high print duty, the toner is fixed onto the toner restricting member of the portion corresponding to a region where such an image is formed.
The present inventors have vigorously examined causes of occurrence of the filming and considered as follows.
That is, when the image of the high print duty is formed, the toner on the developing roller has the heat, the toner is melt-bonded onto the toner restricting member by the heat, and the filming occurs. Thus, on the developing roller with which the toner restricting member is come into contact, in the peripheral portion in which the toner has been melt-bonded, a toner layer which is influenced by such toner and is thinner than the predetermined thickness dimension is formed. The inventors considered that in the toner image which is formed by supplying the toner from such a toner layer, since a desired toner amount is not supplied from the toner layer, a concentration is partially uneven in the print result.
The inventors considered that the reasons why the toner has the heat are as follows.
Although the toner is supplied from the toner layer to the photosensitive drum in accordance with the image to be formed, all of the toner of the layer thickness is not supplied in the supplying portion of the toner layer according to the image. That is, in the supplying portion of the toner layer, only the toner in the upper layer portion is supplied and the toner in the lower layer portion is not supplied but remains. Thus, a difference occurs between the thickness dimension in the toner layer after the toner supply in the portion where the toner supply amount is large and that in the portion where the toner supply amount is small.
In the case where the print duty is high or in the continuous printing, on the other hand, since the toner image is repetitively formed on the photosensitive drum, the reproduction of the toner layer for supplying the toner to the photosensitive drum is repeated.
As mentioned above, the toner is supplied to the developing roller having the toner layer of the uneven thickness dimensions after completion of the toner supply and the toner is restricted by the toner restricting member, thereby reproducing the toner layer. At this time, in the portion of the low thickness dimension in the toner layer, since the number of toner particles is small, each toner particle per unit in such a portion receives a large amount of energy generated by the depression of the toner restricting member. Thus, the inventors considered that the toner particles which received the large amount of energy vibrate among the toner particles and the toner has the heat by frictional heat caused by friction due to the vibration. The inventors, therefore, considered that an increase in toner temperature can be prevented by dispersing the energy which the toner particles per unit receive.